Isohyet

The story of the word

One of the family of map lines that followed Humboldt's isotherm, each built the same way: ἴσος plus whatever is being held equal. ὑετός is a specific word, rain that falls, distinct from ὄμβρος, the downpour or storm. In dry countries the isohyet does real political work: the 400 millimetre line marks roughly where rain-fed wheat stops being possible, and agricultural policy tends to stop with it.

How it travelled

Ancient Greek → scientific coinage → English

First recorded

19th century